The primary thrust of this project is developing new and further defining those animal models already available. The techniques involved include standard mating systems coupled with technological definition as required by each model. Results thus far have been to define the histocompatability (Ag-B) and red-cell (Ag-C) antigens in 18 inbred strains and mutant stocks in the rat. Studies with the nude (athymic) mouse revealed the placental and maternal transfer of thymic activity. The program continues to establish the nude (athymic) gene into 19 inbred mouse strains. Six of the 19 strains have now reached the minimum level of congenicity, i.e., six cycles of outcrossing and backcrossing have been completed.